The disproportionation of toluene involves a well known transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction: ##STR1## Reaction (1) is mildly exothermic.
Mordenite is one of a number of molecular sieve catalysts useful in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite having a network of silicon and aluminum atoms interlinked in this crystalline structure through oxygen atoms. For a general description of mordenite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981, under the heading "Molecular Sieves", Vol. 15, pages 638-643. Mordenite, as found in nature or as synthesized to replicate the natural zeolite, typically has a relatively low silica to alumina mole ration of about 10 or less. However, mordenite catalysts having substantially lower alumina contents are also known. These aluminum deficient mordenite catalysts have silica/alumina ratios greater than 10, ranging up to about 100, and may be prepared by direct synthesis as disclosed, for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al. Both the normal and the aluminum deficient mordenites are known to be useful in the disproportionation of toluene.
The disproportionation of toluene feedstocks may be carried out at temperatures ranging from about 200.degree. C. to about 600.degree. C. or above and at pressures ranging from atmospheric to perhaps 100 atmospheres or above. However, the catalyst itself may impose constraints on the reaction temperatures in terms of catalyst activity and aging characteristics. In general, the prior art suggests the use of relatively high temperatures when employing the high aluminum mordenites (low silica to alumina ratios) and somewhat lower temperatures when employing the low alumina mordenites. Thus, where mordenite catalysts having high silica/alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range. However, U.S. Pat. No. 4,665,258 to Butler et al. discloses disproportionation of a toluene containing feedstock employing an aluminum deficient mordenite catalyst under relatively severe disproportionation conditions; involving a temperature range of 370.degree.-500.degree. C. The mordenite catalysts have silica/alumina mole ratios of at least 30 and, more desirably, within the range of 40-60. The feedstock may be supplied to the reaction zone containing the mordenite catalyst at rates providing relatively high space velocities. The toluene weight hourly space velocity (WHSV) may be greater than 1. Hydrogen is supplied to the reaction zone at a hydrogen/toluene mole ratio within the range of 3-6. The hydrogen pressure may be 500 psi or more. The toluene feedstock need not be dried before supplying it to the reaction zone and the patent discloses toluene feedstocks having water contents in excess of 100 ppm.
Butler et al. also discloses passing a hot preflush gas, nitrogen or hydrogen, to the reaction zone prior to initiating the disproportionation reaction. The preflush gas is heated to a temperature sufficient to strip water from the catalyst so that it is substantially dehydrated by the time the toluene feed is started. This enables the disproportionation process to be carried out initially at a somewhat lower temperature without a reduced toluene conversion than would otherwise be the case. As the disproportionation process continues, the temperatures progressively increase to maintain the toluene conversion at the desired level, typically about 80% of theoretical.
U.S. Pat. No. 4,723,049 to Menard et al. discloses toluene disproportionation carried out over aluminum deficient mordenite of the type disclosed in the aforementioned patent to Butler. In this process, preferably carried out at a reaction zone temperature of 370.degree.-500.degree. C., and more preferably at a temperature of 400.degree.-500.degree. C. with an unpromoted aluminum deficient mordenite catalyst, the supply of toluene to the reaction zone is interrupted while the supply of hydrogen is continued. Preferably the period of interruption during which hydrogen supply is continued is for a period of at least one day before reinstating supply of the hydrogen feedstock to the reaction zone. This mode of operation is disclosed to enhance the aging quality of the catalyst and show a reduction in reactor zone temperature without a corresponding decrease in toluene conversion.
It is also a common practice to promote an aluminum deficient mordenite catalyst with a catalytically active metallic content. For example, U.S. Pat. No. 3,476,821 to Brandenburg et al. discloses disproportionation reactions employing mordenite catalysts having silica/alumina ratios within the range of 10-100 and preferably within the range of about 20-60. The mordenites are modified by the inclusion of a sulfided metal selected from the Group VIII metals. The metal may be included in the mordenite by well known ion exchange or impregnation techniques. The especially preferred sulfided Group VIII metals are cobalt and nickel present in a concentration of 0.5-10 weight percent. When compared with nickel oxide, nickel sulfide is said to provide less overactivity as indicated by gas and saturated hydrogencarbon yield. Here the desired temperature ranges are said to be from about 400.degree. -750.degree. F and preferably 450.degree. -640.degree. F. The metal promoters are said to substantially increase activity and catalyst life, as indicated by runs extending over several hours or days.
As noted previously, hydrogen is supplied along with toluene to the reaction zone. While the disproportionation reaction (1) does not involve chemical consumption of hydrogen, the use of a hydrogen co-feed is generally considered to prolong the useful life of the catalyst, as disclosed, for example, in the above mentioned patent to Brandenburg. The amount of hydrogen supplied, which normally is measured in terms of the hydrogen/toluene mole ratio, is generally shown in the prior art to increase as temperature increases.
Bhavikatti et al., "Toluene Disproportionation Over Aluminum-Deficient and Metal-Loaded Mordenites. 1. Catalytic Activity and Aging", Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 102-105, discloses toluene disproportionation at 400.degree. C. over mordenite catalysts having silica/alumina mole ratios ranging from 12 to 61 at atmospheric pressure and a space velocity (WSHV) of 1. As the silica/alumina mole ratio is increased, catalyst activity is substantially decreased while aging quality is increased. That is, the aging rates were lower. Based upon short term aging studies, the best silica/alumina mole ratio appeared to be 23. Catalyst decay was also suppressed by loading the mordenites with nickel. Mordenites having a silica/alumina ratio of 12, 16 and 23 were modified by the inclusion of nickel by a procedure involving ion exchanging ammonium mordenite with an aqueous solution of nickel nitrate. After ion exchange, the catalyst was activated under a hydrogen environment for two hours. The best activation temperature for nickel modified mordenite having a silica/alumina ratio of 23 was indicated to be about 550.degree. C. The nickel modified mordenite having a silica/alumina ratio of 12 showed significantly lower activity when compared to the nickel loaded mordenite of a silica/alumina ratio of 23.
U.S. Pat. No. 3,562,345 to Mitsche discloses the use of molecular sieves such as mordenite catalysts in the transalkylation or disproportionation of toluene. The catalysts are characterized by a silica/alumina mole ratio from about 6 to about 12, pore openings of from about 3 to about 8 angstroms and the incorporation of catalytically active metallic materials in the oxidized or reduced state, particularly Group VIB and Group VIII metals including molybdenum, tungsten, chromium, iron, nickel, cobalt, platinum, palladium, ruthenium, rhodium, osmium, and iridium. Mitsche discloses transalkylation at temperatures from about 200.degree. C. to about 480.degree. C. and gives specific examples of transalkylation of toluene at temperatures of 420.degree. C. and 450.degree. C.
U.S. Pat. No. 3,677,973 to Mitsche et al., discloses the use of mordenite catalysts composited with an alumina salt providing a silica/alumina mole ratio of about 10 to about 30 in the transalkylation or disproportionation of toluene. The reaction conditions proposed in this patent appear similar to those set forth in the aforementioned Mitsche patent and, like the former patent, Mitsche et al., discloses incorporating Group VIB and Group VIII metals into the catalyst.
U.S. Pat. No. 4,151,120 to Marcilly discloses a process for the manufacture of a hydrocarbon conversion catalyst involving incorporating cobalt, nickel, silver or palladium in a mordenite catalyst having a silica/alumina mole ratio within the range of 10-100. After incorporation of the metal in the mordenite, the catalyst is dried and subjected to a dry calcination procedure at a temperature within the range of 300-700.degree. C. in the presence of an inert or oxidizing gas having a moisture content of less than 1%. Marcilly discloses various examples of the dismutation of toluene under reaction conditions of 420.degree. C., 30 bars, a space velocity (WHSV) of 5 and a hydrogen/hydrocarbon mole ratio of 5.
U.S. Pat. No. 4,723,048 to Dufresne et al. discloses a process for the dismutation of toluene employing a zeolite catalyst modified by the inclusion of metals. The catalyst is described as a sodium containing mordenite in the nature of so-called "wide pores" mordenite, i.e., mordenites who main pores have a diameter of 7-10 Angstroms or "small pores" mordenite, mordenites who main pores have a diameter of 4-6 . Angstroms. The mordenites are treated to extract sodium therefrom to provide not more than 1% by weight sodium ions and preferably not more than 0.5% by weight sodium ions. Dufresne discloses mordenites having silica/alumina ratios of 10.6 (catalyst A), 14.6 (catalyst B), 25.2 (catalyst C), and 58.6 (catalyst D) modified by the inclusion of nickel ranging from 0.43 wt. % to 2.11 wt. % and by the inclusion of nickel with certain other metals. Dufresne discloses activities of the nickel modified catalysts before and after an accelerated aging procedure at conversion rates of 10% and 45%. Catalyst C containing 1.1% nickel showed the best activity with catalyst B containing 2.11% nickel and catalyst D having 0.43% nickel showing slightly lower activities. The poorest activity was with respect to catalyst A having a nickel content of 1.81%.